The present invention relates to frequency variable filters for use in a frequency band such as a microwave band, antenna duplexers, and communication apparatuses incorporating the same.
In a conventionally known band elimination filter, a variable resonance frequency is provided by connecting a reactance element such as a PIN diode or a variable capacitance diode to a resonator via a capacitor, and performing voltage control of the reactance of the reactance element.
FIG. 9 is an electric circuit diagram showing the structure of a conventional frequency variable band elimination filter 1. The filter 1 is composed of resonators 2 and 3, a coupling capacitor C5, capacitors C1 and C2 for making attenuation poles, frequency shifting capacitors C3 and C4, PIN diodes D1 and D2 as reactance elements, inductors L1 and L2 functioning as choke coils, and control-voltage supplying resistors R1 and R2. In addition, the reference numeral P1 denotes an input terminal electrode, the reference numeral P2 denotes an output terminal electrode, and the reference numeral Vc1 denotes a voltage-control terminal electrode.
Although in the conventional frequency variable band elimination filter 1, the attenuation-pole frequency can be voltage controlled, the attenuation bandwidth cannot be varied. Thus, the conventional filter has limited freedom of design.
The present invention, however, can provide a frequency variable filter in which both the frequency of an attenuation-pole and the bandwidth of the attenuation are variable. In addition, the present invention can provide an antenna duplexer and a communication apparatus incorporating the frequency variable filter.
In order to accomplish the above objects, according to a first aspect of the present invention, there is provided a frequency variable filter including a plurality of serial resonance sections each formed by resonators and resonance capacitors electrically connected in series, a coupling element electrically connecting the plurality of serial resonance sections, a plurality of serial circuits composed of frequency shifting capacitors and switching elements, each of the serial circuits being electrically connected in parallel with a respective one of the resonators of the serial resonance sections, and a coupling capacitor electrically connecting the junctions between the frequency shifting capacitors and the switching elements. In such a frequency variable filter, the switching elements may be switched on/off so as to vary the frequency of an attenuation pole, and preferably also the attenuation bandwidth. As the switching elements, for example, PIN diodes or field effect transistors may be used. In addition, as the resonators, dielectric coaxial resonators, distributed-constant lines, or the like, may be used.
With the above arrangement, when the switching elements are switched off, the coupling capacitor and the frequency shifting capacitors influence filter characteristics. As a result, the attenuation-pole frequency becomes higher and the attenuation bandwidth is narrowed. In contrast, when the switching elements are switched on, both ends of the coupling capacitor are grounded, whereby only the frequency shifting capacitors influence the filter characteristics. As a result, the attenuation-pole frequency becomes lower and the attenuation bandwidth is broadened.
Furthermore, in the frequency variable filter, the coupling capacitor electrically connecting the junctions defined respectively between the frequency shifting capacitors and the switching elements may be formed by connection electrodes for the frequency shifting capacitors and/or connection electrodes for the switching elements. With this arrangement, the number of capacitor components can be reduced.
In addition, according to a second aspect of the invention, there is provided an antenna duplexer incorporating the above frequency variable filter.
Furthermore, according to a third aspect of the invention, there is provided a communication apparatus incorporating at least one of the above frequency variable filter and the antenna duplexer. In the antenna duplexer and the communication apparatus, the freedom of design can be increased by using the frequency variable filter.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.